There has long existed a problem that arises during the installation of mechanical actuation systems and involves the need for "rigging" when the actuation system is installed. "Rigging" as used here is a term employed to describe the timing of remotely located actuation components to a power drive unit. Systems that control the position of a device, such as, but not limited to thrust reverser C-ducts and airplane flap surfaces, must continually sense the actual position of that device by some means, such as a feedback arrangement or discrete stops or switches.
In view of the fact that it is very efficient to locate the actuators at the point of movement, and have a single drive unit remotely located, which drive unit is connected to these actuators by devices such as by mechanical shafting, the drive unit must then be timed to the actuators by some means when installed. The drive unit/actuator timing then allows the system to control operation of the flap thrust reversers or surfaces since the limits of travel or positions have been set within the drive unit with respect to the actuators.
A typical situation involving the need for rigging would arise when an actuator is to be installed on a turbofan thrust reverser system. The actuation system must control motion throughout the stroke of the thrust reverser and smoothly stop the thrust reverser door at each end of travel. The conventional approach would be to rig the system by moving the thrust reverser door to a known position, i.e., fully open or fully closed, attach the actuators, run the drive unit to its end of travel and attach the power transmission shafting. If done properly, the thrust reverser can be actuated normally.
Problems arise when errors in rigging occur and the surface, or thrust reverser door slams into their limit of travel before the drive unit reaches its limit of travel. This condition nearly always causes serious damage to the drive system or control surface and is the reason for this invention.
The prior art, typified by the patent to Greenland U.S. Pat. No. 3,655,134, teaches an arrangement that is common and from which arrangement the invention to be described hereinafter makes a fresh departure.
The patent to Greenland uses an actuator feedback to signal snubbing and stopping functions within a power drive unit (PDU). Rigging of a conventional nature as described hereinbefore is required when the actuators are attached to the PDU, since the feedback is rotary and relative in nature until the proper initial orientation is achieved. In Greenland, should the actuators be disconnected from the PDU, and the PDU inadvertently moved through several revolutions and then reconnected to the actuators, the system would be mis-rigged and damage of the type earlier described is likely upon operation. The invention of this specification avoids the possibility of mis-rigging by automatically adjusting a snubbing control valve to a PDU as a direct function of a signal representative of a PDU position commanded and an actual actuator feedback position signal.
There are other patents that may fairly be included as background art and these patents are Capewell, et al U.S. Pat. No. 3,931,944; Aldrich U.S. Pat. No. 4,210,066 and Flippo U.S. Pat. No. 4,191,094. These patents may all be characterized as falling into a single category due to basic similarities within the systems. Each of these patented actuator control systems have revolution counters within the PDU and none have actuator feedback. Accordingly, Capewell, Aldrich and Flippo will inherently operate independent of actuator position and therefore run a number of revolutions in each direction before snubbing and stopping. It should be evident therefore that proper orientation, i.e., rigging of the actuators to the actuators to the PDU is necessary for the PDU starting and stopping points to thereby coordinate the desired actuator starting and stopping points. The invention to be described more fully hereinafter avoids mis-rigging and distinguishes over these three patents in the manner enunciated in respect to the Greenland patent.